Frequency hopping (FH) is a well known spread spectrum technique. Frequency hopping can be used as a multiple access technique in order to share a communications resource among numerous user groups. Since a user group typically employs a unique spread spectrum signaling code, (e.g., frequency hopping set) privacy between individual user groups is easily established.
Local area networks use spread spectrum signaling techniques where a series of user devices, which are typically battery powered, communicate with the access point (AP) which functions as a central controller and may also act as an information relay device. A radio transceiver or wireless adapter within each user device and within the AP provides the radio communication function. The AP typically provides network services such as synchronization, authentication, wireline access, packet relay between user devices, and the like. Communication between user devices may be accomplished directly between user devices or may be accomplished by relaying information from a source user device to the AP and back to a destination user device. The group of user devices and the corresponding AP are referred to as a microcell.
Microcells may also exist that do not have an AP. Such systems are referred to as ad-hoc microcells. For example, a microcell may consist of several portable computers that form a temporary network established for the duration of a meeting in a conference room. In this case, one of the user devices is designated as a master device and is responsible for synchronization between the other user devices.
In a frequency hopping LAN, all devices within one microcell share the same hopping sequence. Each user device changes receiver frequency in unison with all other user devices within the microcell and in unison with the AP such that the change in frequency is virtually transparent to the user devices. The time during which a device is tuned to an individual frequency of the hopping set is referred to as a dwell. The dwell is typically long enough to allow several transmissions of data, referred to as packets, to be transmitted to or from the user devices or the AP.
Many office environments have several microcells operating in close proximity to each other and may be used to provide extended coverage over a large area. In such a situation, microcells must not interfere with neighboring microcells and must facilitate transparent link transfers therebetween. When a user device seeks or requires access to the AP of a different microcell, a link transfer or "hand-off" must be performed between the two microcell controllers (e.g., APs 14). In most cases, however, neighboring microcells employ completely different spread-spectrum signaling codes or hopping sequence sets. As a consequence, acquisition and synchronization are formidable challenges during a hand-off.
If a hand-off is too slow the network may experience various time-outs which may in turn cause the user's session to be dropped, resulting in disconnection from the network. Some systems may even lock-up or experience serious system performance degradation when this occurs.
Typical wireless LANs and devices associated therewith may use a channel access protocol such as carrier sense multiple access (CSMA) to determine a time during which to transmit information, as is known in the art. When two devices transmit at the same time, a collision occurs and neither transmission is successful. The purpose of the CSMA protocol is to allocate the communication channel fairly among the devices to maximize information throughput.
Data or data packets which must be forwarded within the microcell are occasionally much larger than can be efficiently handled by the communication medium. Hence, the AP and the user devices typically employ a packet fragmentation and packet reassembly process. Large packets are broken into a number of smaller pieces known as fragments prior to being transmitted. The fragments are individually transmitted to the destination device(s) where the fragments are then reassembled into the original data packet.
Typical CSMA and CSMA/CA protocols include a strategy for determining which device will access the channel immediately after a transmission has occurred. A common method, known as P-persistence, calls for the time following a transmission to be divided into slots. The duration of each time slot is comparable to the time required for a device to sense or interrogate the state of the channel. In each time slot, a device that wishes to transmit will first sense the state of the channel. If the channel is clear (idle), the device has two choices: 1) the device may begin transmitting with a probability equal to "P", or 2) the device may defer until the next time slot with a probability equal to 1-P. The parameter P is called the persistence probability and is a number greater than zero but less than or equal to one. If the device defers transmission, the decision process is repeated in subsequent time slots until the channel is sensed idle and the device transmits. If the channel is sensed busy, the device must defer its transmission at least until the channel is sensed idle, after which the device can repeat the P-persistence method and try again to access the channel.
A channel is sensed busy if any transmissions are occurring on that channel. As soon as a device which is transmitting on a particular channel ceases transmission, the channel becomes idle. This approach is functional so long as the device which is transmitting information continues to occupy the channel; for as soon as transmission halts, other devices may access the channel.
When the communication system employs frequency hopping, each dwell represents a time when a device may transmit. As is appreciated, however, each dwell utilizes a different channel frequency. Thus, transmission essentially halts when the system hops from a current dwell to a new dwell at a different frequency. Since the channel is deemed idle at the beginning of each new dwell, it would be extremely advantageous to provide a method that maintains channel access priorities across dwell boundaries. It would also be advantageous to provide a method where packet fragmentation and packet reassembly are transparently performed across dwell boundaries.